The invention relates to battery charging, more particularly, the invention relates to improving charge parameters by trimming the charge voltage.
Rechargeable battery packs comprising one or more rechargeable battery cells are widely used to supply power to a variety of portable electronic devices. Rechargeable battery packs save the purchaser of an electronic device the cost of continuously purchasing replacement batteries over the useful life of the device. Additionally, environmental concerns relating to disposal of non-rechargeable batteries points to an increasing use of rechargeable battery packs in the future. A number of chemical compositions have been proposed for rechargeable battery cells including nickel-cadmium (Nixe2x80x94Cd), lithium-ion (Li-Ion), lithium-polymer (Li-polymer) and nickel-metal hydroxide (sometimes referred to as nickel-metal hydride) (Nixe2x80x94MH).
Presently, there are battery charging apparatuses with multiple charging stations or electronic devices with charging bays capable of charging at least one or more battery packs. Typical charging apparatuses have a single charging power source or supply and sequentially charge the battery packs disposed in respective charging stations. This sequential charging method is efficient for Nixe2x80x94Cad cells because these battery cells accept a very rapid charge, that is, if the charger power source can provide enough charging current, an Nixe2x80x94Cd battery cell can be charged in 15 to 20 minutes. However, Li-Ion or Li-Polymer cells require longer charging times and this sequential charging method is not as acceptable to most users of the electronic devices.
More expensive battery charging apparatuses include a separate charging power supply for each of the charging stations. Conceptually, such chargers can be thought of as a plurality of individual chargers supported in a single housing. Multiple power supplies permit simultaneous charging of a battery cell or pack at each charging station thereby reducing total charging time for charging a plurality of batteries. The major disadvantage of such charging apparatuses is the increased expense associated with providing the plurality of power supplies.
Li-Ion battery cells have proven to be very efficient in terms of watt-hours per unit volume of the cell and high output voltage generated by the cell. Additionally, Li-Ion cells exhibit a long useful life. Thus, battery packs comprising one or more Li-Ion battery cells have found increasing use in a variety of portable electronic devices requiring a compact, higher voltage power supply such as portable computers, cell phones, digital cameras, and personal data assistants. For example, the Model No. 18650 Li-Ion battery cell available from Toshiba has a 4.2 volt output voltage potential when the cell is fully charged. The cell is considered discharged when the output voltage potential drops to about 2.6 volts. During charging of a Li-Ion battery cell, the charging current through the cell must be limited. In the case of the Toshiba Li-Ion cell, charging current must be limited to a maximum of 1.25 Amps, otherwise damage to the cell may result. In addition, the cell has a recommended charge voltage that must not be exceeded for safety reasons.
In typical applications, two or more Li-Ion battery cells are disposed in a casing and electrically coupled in series and parallel to form a rechargeable Li-Ion battery pack. The battery pack has an aggregate maximum charging current based on the number of cells in parallel and an aggregate recommended charge voltage based on the number of cells in series. The battery pack is sized to fit in a battery pack receiving opening or bay of an electronic device. During use of the device, the battery pack gradually discharges as it supplies power to the electronics of the device. When the power supplied by the pack falls below a threshold value, the pack is either recharged in the device itself or removed from the device and recharged using a recharging apparatus.
A discharged Li-Ion battery cell has a very low charging impedance. Therefore, when such a cell is first inserted or docked in a charging station of a charging apparatus, the charging current must be limited to a maximum charging current for that cell to avoid damaging the cell. During the charging process, chemical reactions within the Li-Ion cell cause the charging impedance of the cell to increase. As the cell charging impedance increases, the applied voltage will increase proportionately with cell charging impedance until the applied voltage equals the recommended charge voltage for the cell. The time required for the charging voltage to reach the recommended charge voltage is referred to as the crossover time.
After the crossover time is reached, the charging apparatus limits the applied voltage to the recommended charge voltage to avoid damaging the cell. Since the battery charging impedance continues to increase at a decreasing exponential rate as the cell charges, charge current decreases exponentially until the cell is fully charged. For a typical Li-Ion 4.2 volt cell, crossover time is achieved after about 45 minutes while full charging requires about three hours.
In order to minimize total charging time for a rechargeable battery pack, the charging apparatus preferably provides as much charging power as possible during the period up to the crossover time while making sure that the aggregate maximum charging current limit for the battery pack is not exceeded. For a battery pack comprising six typical Toshiba Li-Ion cells electrically connected in a 3-series by 2-parallel arraignment, the values for the maximum charge current and the aggregate recommended charge voltage are 2.5 amps and 12.6 volts respectively.
Rechargeable battery packs are utilized extensively in powering a variety of portable electronic devices. One such device is the portable computer where the battery functions to provide power to the various circuitry therein. Of course, the battery requires charging periodically in order to provide the power required from it. Several current battery charging apparatus have two or more charging circuits or algorithms available so that the battery may be charged more rapidly or more slowly, depending on the other functions required of the portable computer by a user. While rapid charging of a battery is generally preferred in that it drastically reduces the time required for charging, it has been found that having an option with respect to the charging rate to reduce the power consumed is useful in preventing overheating of the battery or the portable computer during charging and otherwise maximizing the effectiveness of the battery in the portable computer.
In light of the foregoing, a primary objective of the present invention is to provide an apparatus and method for enabling rapid charging of a battery or for reducing the peak power required during charging in an electronic device or a stand-alone battery charger.
A charger for a battery includes a power source and a control circuit. The battery has an internal resistance, at least one cell having a recommended charge voltage, and an aggregate recommended charge voltage. The power source is capable of providing an applied voltage and charge current to the battery. The control circuit is connected to the power source and is capable of detecting the applied voltage of the battery and enabling the charge current and applied voltage to the battery until the applied voltage is greater than the aggregate recommended charge voltage by an amount less than the product of the internal resistance and the charge current. The control circuit is capable of reducing the applied voltage towards the aggregate recommended charge voltage at a controlled rate such that the recommended charge voltage of the at least one cell is not exceeded.